For repairing or remedying hydroxyapatite structures such as dentin, (meth)acrylic resins for biochemical art (i.e., acrylic resins) having radical polymerizability have been conventionally employed. In such use application, it is required that an adhesive strength between the acrylic resins and the hydroxyapatite structures hardly varies for a very long period of time and the adhesive strength therebetween is high. Therefore, in the case of remedying dentin by the use of a remedying acrylic resin containing a methacrylic resin as an adhesive component, there has been employed a method comprising treating the beforehand-abraded (or beforehand-shaved) surface of the dentin with an etching agent such as an acid to remove shavings and to roughen the surface, then coating the surface with an adhesive containing a similar component to that of the remedying acrylic resin, then curing the coated adhesive, and coating the cured adhesive with the remedying acrylic resin.
However, the etching agent used in the roughening treatment, such as an acid, sometimes permeates inside of the aimed object to damage the hydroxyapatite structure, because the etching agent is a low-molecular compound. Further, if the hydroxyapatite structure is dentin, the etching agent permeates the structure to stimulate a nerve, and therefore the patient sometimes suffer an acute pain in the remedy.
In the conventional remedy for such parts, a cement prepared by combining a polyacrylic acid with zinc oxide or an aluminosilicate glass is applied on a surface of the hydroxyapatite structure and then the acrylic resin is applied thereon, to mitigate the stimulation in the adhering stage. However, such cement inherently has low adhesion properties, so that it is impossible to make a sufficient remedy by the use of the cement.
For the above-mentioned reason, the present inventors have proposed to employ a polymer-type surface-treating agent comprising an acidic group-containing polymer, in order to reduce a damage of the hydroxyapatite structure and to prevent the etching agent from permeation inside of the aimed object. (see: Japanese Patent Application No. 60(1985)-171024)
The acrylic copolymer described in this specification has a sulfonic acid group. Concretely, a copolymer of a methacrylic ester (e.g., methyl methacrylate) and a monomer containing a sulfonic acid group (e.g., a p-styrene sulfonic acid) is dissolved in a solvent such as water or ethanol, then the resulting solution is applied on a surface of the hydroxyapatite structure, and thus coated structure is washed with water. By the adaptation of this method, the surface treatment of the hydroxyapatite structure can be done with restraining invasion of the etching agent into the hydroxyapatite structure. Further, on the surface of the hydroxyapatite structure is formed a layer which is adsorbed with a copolymer used in this treatment. The copolymer layer firmly adheres to the hydroxyapatite structure, because the sulfonic acid group of the copolymer is bonded to the calcium component contained in the hydroxyapatite structure. Moreover, the copolymer layer has an excellent affinity for the adhesive acrylic resin (or remedying acrylic resin) which is to be applied on the layer, because the copolymer of the layer contains recurring units derived from methyl methacrylate Accordingly, in the case of using the acrylic copolymer, the hydroxyapatite structure and a filler are united with each other via the acrylic resin, and thereby the adhesion between the hydroxyapatite structure and the filler becomes excellent and this excellent adhesion can be kept for a long period of time.
However, the present inventors have further studied on the acrylic copolymer, and found that the affinity of the acrylic copolymer for the acrylic resin which is to be applied on the copolymer should be more improved although the adhesion of the copolymer to the hydroxyapatite structure is prominently high. For example, the adhesive strength of the copolymer varies depending upon a polymerization initiator contained in the acrylic resin.
If the content of the recurring units derived from a monomer having an excellent affinity for resins such as methyl methacrylate is increased by the conventional method in order to solve the above-mentioned problem, the resulting acrylic copolymer tends to lower in solubility in solvents and also tends to lower in adhesive strength to the hydroxyapatite structure. That is, it has been found that the aimed adhesive strength of the acrylic copolymer cannot be obtained by only adjusting the copolymer composition, namely, a ratio between methyl methacrylate and p-styrene sulfonic acid.
The object of the present invention is to provide a novel acrylic copolymer, a process for the preparation of the acrylic copolymer and use application of the acrylic copolymer.
In more detail, the object of the invention is to provide a novel acrylic copolymer which has excellent adhesion properties to both the hydroxyapatite structures and the acrylic resins containing an acrylic ester composition of radical polymerizability as a host adhesive component and also shows high solubility in solvents such as water and ethanol. Further, the object of the invention is to provide a process for the preparation of the acrylic copolymer and use application of the acrylic copolymer.